Kelps in southern Africa are thriving, but some key inhabitants of kelp forests are not

15 March 2017 | John Bolton, Professor of Biology and a Marine Plant Biologist, University of Cape Town and Laura Blamey, Marine Ecologist in the Department of Biological Sciences, University of Cape Town

There’s a forest in the extreme west of South Africa which very few people know much about. It’s 1000km long but only around 100m wide, and also extends along the rocky coastline of Namibia. This forest is a kelp forest. Kelps are large brown seaweeds and there are two main species in southern Africa, Ecklonia maxima and Laminaria pallida. These form the canopy, and they are mostly a few metres tall, although Ecklonia can reach 17m.

……while far below

The sea-blooms and the oozy woods which wear

The sapless foliage of the ocean, know

Thy voice, and suddenly grow gray with fear

PB Shelley (Ode to the West Wind)

The forest is as productive per unit area as a tropical rain forest, and has a variety of diverse organisms. Many of the people who live on its fringes provide for themselves by extracting and selling food items from it, in a sense being modern “hunter-gatherers”. Some of these resources that live within kelp ecosystems are almost gone, and much of what is left is now collected illegally.

Around a quarter of the worlds coastlines, along cool-water rocky coasts, are populated by kelp forests. With climate change, they are on the move. As a result, a working group was formed by the National Center for Ecological Analysis and Synthesis to study these changes on a global scale. The study showed that kelp abundance is declining in 38% of world regions, but increasing in 27%. This means that almost two-thirds of global kelp forests are changing.

Seawater temperature is critical for kelp survival, and the cooler the water the higher the amounts of dissolved nutrients necessary for kelp growth. Global seas are reacting to climate change differently on different continents. In Europe, considerable warming is causing kelp forests to disappear in the south, like Portugal for example, but increase in abundance towards the Arctic. But in southern Africa, for example, kelps are thriving.

While kelps themselves are doing quite well, some of their inhabitants are not.

Not the same everywhere

Perhaps the most severe effects of climate change on kelp are in Australia. Here, the edges of the kelp-dominated Great Southern Reef have seen considerable diebacks of kelp due to large warming events. These are also associated with southward movement of kelp-eating tropical fish and urchins.

In the extreme south of Australia, in Tasmania, beautiful forests of the giant kelp Macrocystis pyrifera have nowhere to go following considerable warming and increased grazing. And its seems likely they will soon cease to exist.

In South Africa, kelps are having a more productive time, for now. If there was a poem on these forests, it should perhaps be called “Ode to the Southeast Wind”. These winds, which cause large-scale upwelling of cool, nutrient-rich water, have been increasing in intensity and duration in recent years in southwestern South Africa.

There’s evidence that kelp forests have become more abundant at the southern end of their range, and also a dispersal event around 2006 meant that a kelp forest of Ecklonia maxima appeared at De Hoop Nature Reserve, around 70km east of where forests previously grew.

Sea water climate change scenarios suggest that further north, in Namibia and the Northern Cape province of South Africa, warming is occurring and conditions may become gradually less favourable for kelp forests in this region.

Ripple effect

Kelps in South Africa are actually doing quite well because of cooling waters and increased nutrients. But some of the major iconic species of kelp ecosystems, like lobster and abalone are not doing well. This could be for a number of reasons such as fishing and climate change.

Major marine resources that live in these kelp forests are abalone (Haliotis midae) and rock lobster (Jasus lalandii), and they too have undergone notable changes, and unlike kelp, are not doing so well. Both abalone and rock lobster are now in great trouble due to overfishing, with populations estimated at a few percent of original abundances. In addition, shifts in the abundance of some species (rock lobster) have seriously impacted other species (abalone).

In the early 1990s a major eastward shift brought lobsters east of False Bay, while illegal fishing of abalone was on the rise. This resulted in considerable changes in the kelp forests, given that lobsters are predators that eat major kelp grazers, especially the spiny sea urchin (Parechinus angulosus), but also juvenile abalone that hide beneath the urchin spines. Following a decline in these grazers, due to lobster predation, kelps and seaweeds became more abundant, while many animal groups, that are eaten by lobsters, were reduced massively.

The kelp forests of South Africa were extensively studied in the 1970s, but little biological study has gone on since then. There’s a dire need to understand the country’s altered kelp forests; how they work, the effects of environmental change, and the dynamics of these changed systems, with new patterns of dominance of different species.

Image by NOAA's National Ocean Service, Flickr.

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